1. Field of the Invention
The present invention generally relates to body stimulators and, more particularly, to an improved body tissue stimulator of the unipolar type which is rechargeable by a magnetic field.
2. Description of the Prior Art
Although the invention relates to various types of body tissue stimulators which are implantable in the body, it will be described in connection with a cardiac stimulator, generally referred to as a pacemaker, for explanatory purposes only, rather than to limit the invention thereto.
As is appreciated, a pacemaker is a device which applies stimulating pulses to the heart. In recent years great advances were made in the development of such pacemakers. At present a pacemaker is available which is implantable in the body and which includes a source of energy, such as a battery, which is rechargeable by an external alternating magnetic field, so that the pacemaker does not have to be removed periodically from the body for battery replacement, thus eliminating the need for frequent surgical operations. One available pacemaker is sealed in a hermetic container so as to extend the life of the pacemaker. During recharging the external magnetic field passes through the hermetic container and is picked up by a pickup coil, which forms part of the recharging circuitry inside the hermetic container.
The aforedescribed hermetically-sealed rechargeable pacemaker is of two types. These include the bipolar type and the unipolar type. As is known in the bipolar type, two electrodes extend from the pacemaker to the heart and each pulse is applied between them. In the unipolar type only one electrode extends to the heart. The other electrode is in the form of an outer electrically conductive metal plate on the side of the pacemaker. The body saline solution and body tissue, which are electrically conductive, provide the return electric path between the electrode in or on the heart and the outer plate, which is electrically connected to the circuitry inside the hermetic container. The outer plate is usually in direct contact with the skin or other body tissue inside the body. One example of a prior art unipolar pacemaker is described in U.S. Pat. No. 3,735,766. In the prior art the outer electrode is formed of a continuous sheet of a biocompatible metal of a sufficiently large area, e.g., several square inches in order to minimize the current density thereat.
In the rechargeable unipolar pacemaker the recharging alternating magnetic field is provided from an external recharging head, which is placed against the body near the location of the implanted pacemaker. Quite often the head is located with respect to the pacemaker so that adequate recharging takes place while part of the magnetic field passes through the outer plate.